Stream cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace wherein the feedstock, typically comprising crude or a fraction thereof optionally desalted, is heated sufficiently to cause thermal decomposition of the larger molecules. Among the valuable and desirable products include light olefins such as ethylene, propylene, and butylenes. The pyrolysis process, however, also produces molecules that tend to combine to form high molecular weight materials known as steam cracked tar or steam cracker tar, hereinafter referred to as “SCT” or simply “tar”. SCT is also known in the art as “pyrolysis fuel oil”. These are among the least valuable products obtained from the effluent of a pyrolysis furnace. In general, feedstocks containing higher boiling materials (“heavy feeds”) tend to produce greater quantities of SCT.
SCT is among the least desirable of the products of pyrolysis since it finds few uses. SCT tends to be incompatible with other “virgin” (meaning it has not undergone any hydrocarbon conversion process such as FCC or steam cracking) products of the refinery pipestill upstream from the steam cracker. At least one reason for such incompatibility is the presence of asphaltenes. Asphaltenes are very high in molecular weight and precipitate out when blended in even small amounts into other materials, such as fuel oil streams.
One way to avoid production of SCT is to limit conversion of the pyrolysis feed, but this also reduces the amount of valuable products such as light olefins. Another solution is to “flux” or dilute SCT with stocks that do not contain asphaltenes, but this also requires the use of products that find higher economic value in other uses.
Many low-volume uses of SCT have been devised. For instance, U.S. Pat. No. 4,207,168 teaches making needle coke from pyrolysis fuel oil by separating quinoline insolubles and asphaltenes from the fuel oil and subjecting the remaining portion to coking
In U.S. Pat. No. 4,207,168, a pyrolysis fuel oil is contacted with a promoter liquid to separate quinoline insolubles and asphaltenes from the fuel oil. The fraction free of quinoline insolubles and having a reduced content of asphaltenes is optionally subjected to coking to produce needle coke or employed directly for the production of carbon black.
In U.S. Pat. No. 4,446,002, the precipitation of sediment in unconverted residuum obtained from a virgin residuum conversion process is taught to be suppressed by blending the unconverted residuum with an effective amount of a virgin residuum having an asphaltene content of at least about 8 wt % of the virgin residuum at a temperature sufficient to maintain both residuum components at a viscosity of no greater than about 100 cSt (centistokes) during blending. Virgin residuum is the bottoms product of the atmospheric distillation of petroleum crude oil at temperatures of about 357 to 385° C.
In U.S. Pat. No. 5,443,715, steam cracked tar is upgraded by mixing with a “hydrogen donor”, preferably hydrotreated steam cracked tar, at or downstream of quenching of the effluent of a gas oil steam cracker furnace. In this regard, see also U.S. Pat. Nos. 5,215,649; and 3,707,459; and WO 9117230.
Other references of interest include U.S. Pat. Nos. 3,622,502; 3,691,058; 4,264,334; WO 91/13951; DE 4308507; and JP 58-149991.
Despite these advances, there remains a problem that SCT continues to be generated in amounts beyond the capacity of current technology to be efficiently utilized. Thus, significant amounts of SCT must be disposed of by adding to fuel oil pools or simply local combustion to generate, for example, steam. However, steam cracker tar, even relatively low asphaltene steam cracker tar, is generally incompatible with fuel oil pools such as Bunker C fuel oil. Onsite tar burning in site boilers is then preferred to avoid tar separation investment, but tighter emission regulations increasingly limit the amount that can be burned for this purpose.
The present inventors have discovered a process for comprising deasphalting tar that is conveniently integrated with one or more pyrolysis furnaces, the process providing an efficient method of upgrading tar.